Got a Craftsman R1000 30" 6 speed rear engine. Great little mower for my application. When ever I move the deck height to the 3rd position and engage the blades, the engine spits and sputters and complains. Put the deck height to the 4th position engage the blades, the engine pretty much stalls out untill I disengage the blades. Deck height positions 1,2 and 5 (which I seldom use), the thing runs like a charm. Any idea on what to check to try and resolve this.